Breast cancer (BC), a genetically heterogeneous disease, is the most commonly identified malignant disease in Western women after non-melanocytic skin cancer. It attacks one in eight women (~12%), impacting nearly every family worldwide, including the US Veterans population. Approximately 30% of those diagnosed will develop the invasive form of the disease, which is ultimately incurable. Therefore, it is a major health issue for women Veterans. Although increased early diagnosis and new therapeutic regimens have significantly improved BC survival, the therapeutic options for advanced stage BC are limited. One reason these regimens are not effective is that they do not target the tumor microenvironment, which plays critical roles in tumor progression. Thus, there is a need to understand the etiology of BC progression from a non- invasive microenvironment and to use this awareness for the design of a targeted, molecular based therapy. We recently discovered that the matricellular protein CCN5 is highly expressed in non-invasive BC cell lines and tissue samples, as compared to invasive ones and plays a negative regulator of plasticity in vitro. It prevents the epithelial to mesenchymal transition (EMT) process in BC cells. Furthermore, preliminary studies suggest that CCN5 may inhibit the transition of in situ ductal carcinoma (DCIS) to invasive BC through the protection of biological fences (myoepithelial layer and basement membrane), or may prevent or reduce the growth of aggressive BC cells or both and possibly make ER negative aggressive BC cells sensitive to hormone therapy. Now, we propose to establish the above premises and unravel the mechanisms of CCN5 in regulation of cancer cell progression to invasion using a xenograft model, genetically engineered mouse models, genetically manipulated human BC cell lines and stromal cells. To test this hypothesis, three specific aims are proposed: Aim 1: We will determine whether CCN5 is able to prevent DCIS to invasive ductal carcinoma transition by protecting the myoepithelial layer or basement membrane degradation or both. To test this, conditional knock- down strategies will be used in a MCFDCIS-intraductal-xenograft model (MIND model). Aim 2: We will determine the effect of CCN5 on tumor progression and survival in BC. To test this, we will use the MMTV-neu/Tet-op-MMTV-CCN5 (MNIC5) transgenic mouse model to evaluate the impact of CCN5 gains in HER-2/neu driven mammary tumorigenesis. Aim 3: We will determine whether the gain of CCN5 re-sensitizes antiestrogen's action on BC. Both in vitro and in vivo (xenografts and MNIC5 mouse model) will be used. We will use state-of-the art techniques, our development of a Tet-op-MMTV-CCN5 mouse model, and the unique collective expertise of our multi-disciplinary team to uncover the role of the CCN5 pathway in BC progression. Significance: The proposed studies should result in new explanations of the protective role of CCN5 signaling in microenvironment of invasive cancers. Moreover, this study should clarify the functional roles of CCN5, while revealing novel targets and pathways which will aid in our research goals of finding effective therapeutic reagents to battle breast cancer bringing an improved prognosis to US Veterans and other pre- and post- menopausal patients.